1. Field of the Invention
This invention relates to automatic depth gain compensation in an ultrasound imaging system.
2. Description of Related Art
Ultrasound imaging systems generally operate by transmitting ultrasound signals from an ultrasound transducer or a plurality of transducer elements into a human body at a skin surface or within a body cavity, and receiving ultrasound signals reflected by objects or structures, such as organ tissue or other acoustic interfaces in a scan region (such as a scan plane) for the ultrasound signals, back to the ultrasound transducer. The reflected ultrasound signals are processed, and the processed ultrasound signals are displayed on a monitor or other display device for an operator to review. In general, objects or object boundaries, such as tissue or other acoustic interfaces, that reflect more energy may be shown more brightly on the display, while objects that reflect less energy may be shown less brightly on the display.
One problem that has arisen in the art is that ultrasound signals are generally attenuated as they penetrate into the region sought to be imaged, and further attenuated as they return after reflection. This "range attenuation" is in addition to known other sources of attenuation that arise due to the operation of the ultrasound system and which can be compensated for, such as energy dissipation away from a focal point. Moreover, range attenuation due to tissue penetration may be difficult or impossible to account for ahead of time, such as that due to voids (e.g., heart chambers or cysts), density changes, or other aspects of the geometry of the tissue or structures being imaged. If range attenuation is not compensated or is compensated incorrectly, the displayed ultrasound image may appear to wash out for greater ranges, may yield non-uniform contrast for some organ tissue, or may otherwise fail to properly represent organ tissue or other structures in the region sought to be imaged.
One method of addressing range attenuation has been to provide a set of slider potentiometers, or other manually operated controls, for the operator to adjust image gain for a set of range bands. For example, the imaging system may provide the operator with a set of eight sliders, each one for adjusting the image gain within a corresponding one of a set of eight range bands. This adjustment is sometimes called "depth gain compensation" or "time gain compensation" (because greater depth implies greater time for the ultrasound signals to reach a reflecting object or interface and return to the ultrasound transducer). Preferably, the operator should adjust the controls so that all regions of a uniform tissue in a field of view would have uniform brightness.
While this method of addressing range attenuation generally achieves the purpose of allowing the operator to manually compensate for range attenuation effects which cannot otherwise be anticipated by the ultrasound system, it suffers from several drawbacks. One drawback is that the operator does not know the actual attenuation due to range in advance of reviewing the image, and so may overcompensate or undercompensate from lack of knowledge. Similarly, upon reviewing the image, the operator may inaccurately compensate for actual image brightness in some bands, causing uneven displayed tissue brightness when that tissue extends across different range bands, and possibly other erroneous presentation of displayed tissue brightness. Another drawback is that the range bands for which the operator is able to adjust the gain are relatively wide, while the gain selected for each band is typically applied linearly throughout the entire width of a range band. Therefore, the operator may be unable to adjust the controls finely enough to adequately compensate for local range attenuation effects. Providing a larger number of range bands allows the operator finer control, but also increases the work the operator must do. It is therefore desirable to automate the procedure for depth (range) gain compensation. Still another drawback is that a changing image may require continual adjustment by the operator.
One method for automating range attenuation was shown in U.S. Pat. No. 4,662,380, titled "Adaptive Time Gain Compensation System for Ultrasound Imaging", issued May 5, 1987, in the name of James K. Riley. In this method, each pixel of the previously displayed image is processed to determine a histogram of intensity values for the pixels in the previously displayed image. The method includes a search for a set of peaks in the histogram band, so as to determine an automatic gain compensation curve in response to those peaks. This method is based on the concept that acoustic signal data will always produce a histogram in which a peak corresponds to a class of objects or structures: dark pools (such as blood), objects and structures of interest (such as organs and other tissue of interest), and bright reflectors (such as bony material or hard cysts). U.S. Pat. No. 5,313,948, titled "Ultrasonic Diagnostic Apparatus", issued Feb. 24, 1994, in the name of Masaru Murashita, et al., depends upon computing histograms for a set of several image depths; the peaks of the histograms are also used in the method disclosed in that patent to estimate image attenuation, and suffers from similar drawbacks, as shown herein below.
While these histogram-oriented approaches to range attenuation estimation generally achieve the purpose of automatic gain compensation responsive to depth or range, they suffer from the drawback that the histograms determined by these methods are potentially unreliable. First, the expected multiple histogram peaks do not always occur in practice; there may in some cases be no readily identifiable histogram peaks. Second, even when the expected histogram peaks do occur and can be discerned, they do not always fairly represent the brightness level to which automatic gain compensation should be applied.
Another method for automating range attenuation estimation is shown in U.S. Pat. No. 4,852,576, titled "Time Gain Compensation for Ultrasonic Medical Imaging Systems", issued Aug. 1, 1989, in the name of Dan Inbar, et al. In this method, scan-converted pixel data are divided into a sequence of range (depth) bands, and each depth band is averaged over the entire cross-range direction, to produce a sequence of values intended to represent average attenuation for that depth band. A set of straight line segments are fit to sections of the averaged attenuation values to determine an attenuation curve; the inverse of this attenuation curve then determines a computed gain curve.
While the method of U.S. Pat. No. 4,852,576 of addressing range attenuation generally achieves the purpose of automatic gain compensation responsive to depth (range), it suffers from the drawback that the averages determined by this method are computed over the entire cross-range extent of the data, and are thus likely to be erroneous. First, these averages are likely to include many regions whose attenuation at the selected depth band is outside the image nominal brightness range, such as dark pools and bright reflectors. Second, these averages are determined without regard to cross-range gain rolloff at left and right edges of the image.
Another problem with the method of U.S. Pat. No. 4,852,576 for automatic depth gain compensation is that many computations on individual pixels of scan converted data are required, thus generating a need for extensive computational power. If a general purpose processor is used, this can lead to a very large time delay between the acquisition of the ultrasound acoustic signal and the determination and display of the gain compensated ultrasound acoustic signal. This latency makes it difficult to adjust a dynamically changing displayed image. Alternatively, if specialized hardware is designed to provide the automatic depth gain compensation, this can lead to an inflexible system that cannot easily be upgraded or otherwise modified.
Another problem with prior art automatic depth gain compensation systems is their failure to remove any previously applied gain compensation values from the acoustic signal data. This can lead to instabilities in automated applications of gain, particularly when the image intensity is dynamically changing.
Another problem with prior art automatic gain compensation methods is their failure to account for cross-range gain rolloff at the image edges.
Still another problem with prior art automatic gain compensation methods is the absence of means for responding to operator preferences for modifying gain compensation for extreme ranges.
It would also be advantageous to provide compensation for differences in image brightness that vary with the azimuth of the ultrasound signal data. Known methods of automatic adjustment of image gain, such as those cited herein above, do not perform azimuth gain compensation or any other kind of cross-range gain compensation. Moreover, if prior art methods of automatic depth gain compensation were somehow used instead to accomplish cross-range gain compensation, the cross-range gain compensation would also suffer from the same drawbacks as for depth gain compensation.
Accordingly, it would be advantageous to provide a system for automatic gain compensation, depth gain compensation, cross-range gain compensation, or a combination thereof, to supplement or to be used in lieu of gain compensation made by an operator.